1. Field of the Invention
The present invention relates in general to a method for generating dummy frequencies to enable a smooth frequency hard handover in a digital code division multiple access (CDMA) mobile communication service when base stations are configured for different frequencies, and more particularly to a dummy frequency generation method for making the coverage of a dummy frequency the same as that of a service frequency using basic signals of the service frequency.
2. Description of the Prior Art
In a Code Division Multiple Access (CDMA) system, a service frequency includes a plurality of channels which are identified according to codes. A forward link from a base station to a mobile station is composed of pilot, sync, paging and traffic channels. The pilot channel transmits information allowing the mobile station to acquire the system and identify the base station. The coverage of the base station is determined according to a ratio of chip energy of the pilot channel to interference density.
The interference density is determined based on the sum of power of the sync, paging and traffic channels and ambient noise power of the mobile station. For this reason, the coverage of a base station varies according to a power ratio of the channels in the service frequency and a traffic load.
In mobile communication systems such as cellular, personal communications service satellite mobile communications and future public land mobile telecommunications systems, handover must be performed to maintain a call when a mobile station passes through the boundary between base stations. Particularly, in a CDMA mobile communication system, when service frequencies of two base stations are the same in number and value, a call can be continuously maintained by soft handover with no frequency conversion. However, when the service frequencies of the two base stations are not the same in number, a frequency conversion may be required to maintain the call, with an engaged mobile station which is called "frequency hard handover".
When the service frequencies of the handover target base station are smaller in number than those of the adjacent base station, the handover target base station adds dummy frequencies or pilot beacons to service frequencies to make the number of the frequencies the same as that of the adjacent base station. In this manner, a mobile station using a frequency other than the service frequency of the handover target base station obtains information of the handover target base station to perform frequency hard handover. At this time, each dummy frequency is composed of a part or all of a pilot channel signal, a sync channel signal and paging channel signals. Here, the term "dummy frequency" represents all of pilot beacons or dummy pilots, each of which transmits only the pilot channel signal and dummy frequencies, each of which is composed of a part or all of the pilot channel signal, sync channel signal and paging channel signals.
FIG. 1 is a block diagram illustrating the construction of a system for performing a conventional dummy frequency generation method. The base station is provided with a service frequency with traffic channels and dummy frequencies with no traffic channel. The service frequency includes a pilot channel signal A.sub.1, a sync channel signal B.sub.1, a paging channel signal C.sub.1 and traffic channel signals D.sub.1 -D.sub.N. Channel elements 1-5 perform digital signal processes including frequency spreading, respectively, with respect to the pilot channel signal A.sub.1, sync channel signal B.sub.1, paging channel signal C.sub.1 and traffic channel signals D.sub.1 -D.sub.N. The processed signals are combined by a power combiner 6 and converted into a radio frequency signal F.sub.1 by a transmitter/receiver unit 7. Then, the radio frequency signal F.sub.1 is amplified by a linear amplifier 8 and supplied to an antenna (not shown). Here, the pilot channel signal A.sub.1, sync channel signal B.sub.1, paging channel signal C.sub.1 and traffic channel signals D.sub.1 -D.sub.N are adjusted in gain by the channel elements 1-5. The pilot channel signal A.sub.1, sync channel signal B.sub.1 and paging channel signal C.sub.1 are typically maintained at gains set upon initialization of the base station. However, the traffic channel signals D.sub.1 -D.sub.N are dynamically varied in gain to maintain a call quality depending on a variation in traffic amount. As a result, the base station coverage, or a ratio of chip energy of the service frequency pilot channel to noise density, is varied.
For the smooth frequency hard handover, a handover target base station must have N-1 dummy frequencies in addition to one service frequency when the adjacent base station has N service frequencies. Each dummy frequency is the same in construction as the service frequency, with the exception that it has no traffic channel signal. Pilot, sync and paging channel signals of the first dummy frequency are processed respectively by channel elements 9-11. Similarly to those in the service frequency, the processed signals are combined by a power combiner 12 and converted into a radio frequency signal F.sub.2 by a transmitter/receiver unit 13. Then, the radio frequency signal F.sub.2 is amplified by the linear amplifier 8 together with the radio frequency signal F.sub.1 of the service frequency and supplied to the antenna. Channel elements 14-16 correspond to the N-1th dummy frequency. Similarly, pilot, sync and paging channel signals of the N-1th dummy frequency are processed respectively by the channel elements 14-16. The processed signals are combined with a power combiner 17 and converted into a radio frequency signal F.sub.N by a transmitter/receiver unit 18. Then, the radio frequency signal F.sub.N is amplified by the linear amplifier 8 together with the radio frequency signal F.sub.1 of the service frequency and the radio frequency signal F.sub.2 of the first dummy frequency and supplied to the antenna.
In the above-mentioned conventional system, the coverage of the service frequency F.sub.1 is determined according to a pilot chip energy-to-noise density ratio at the output of the power combiner 6. The noise density is determined based on the sum of power of the sync B1 and paging C1 channel signals from the channel elements 2 and 3 and power of the traffic channel signals from the channel elements 4, . . . , 5. For this reason, the coverage of the service frequency F.sub.1 is varied depending on a load amount on the traffic channels. However, the coverage of each of the dummy frequencies F.sub.2, . . . , F.sub.N is determined according to a pilot chip energy-to-interference density ratio at the output of a corresponding one of the power combiner 12, . . . , 17. The interference density is determined based on power of the sync and paging channel signals from corresponding ones of the channel elements 10 and 11, . . . , 15 and 16. As a result, the coverage of the dummy frequency is always constant, an necessarily different from the varying coverage of the service frequency.
In other words, the coverage of each of the dummy frequencies must be the same as that of the service frequency to make frequency hard handover smooth. However, when the coverage of a dummy frequency of a handover target base station is larger than that of a service frequency thereof, a handover executing mobile station receives the signal of the dummy frequency of the handover target base station at the boundary of the dummy frequency coverage of the handover target base station and changes it communication frequency to the service frequency of the target base station. In this case, a call is dropped because the service frequency coverage of the handover target base station is small. Also, because the dummy frequency coverage of the handover target base station is extremely large, the interference to the adjacent base stations is increased, resulting in a reduction in the coverage of a service frequency of the adjacent base stations. On the other hand, when the coverage of the dummy frequency of the handover target base station is smaller than that of the service frequency thereof, the handover executing mobile station cannot obtain information of the handover target base station at the boundary of the service frequency coverage of the adjacent base station. As a result, the frequency hard handover cannot be performed.